Data storage devices employing rotating magnetic or optical media disks are known for high capacity, low cost storage. Such disks typically have a multiplicity of concentric data track locations, each capable of storing useful information. The information stored in each track is accessed by a transducer head which is moved among the tracks during track seeking operations and which is maintained in alignment with the track during read only and/or read/write track following operations of the device. The electro-mechanical assembly for rotating the disk relative to the head and for moving the head radially relative to the disk for track accessing purposes is known as the head and disk assembly (HDA). A control mechanism is provided in order to maintain the head within the boundaries of each data track, and may take the form of detents provided by a stepping motor, or by a continuously positionable actuator operating within a closed loop servo, or time sampled servo.
Until recently, most low cost, non-removable rigid magnetic media disk data storage devices utilized standard interfaces for connection of the HDA to a controller. The typical standard applicable to five and one quarter inch rotating fixed disks was established in the 1980 time frame by Seagate Technology, Inc., of Scotts Valley, Calif. That standard has been variously referred to as the "ST 506" as well as the "ST 412" interface. "ST 506" refers to the original five megabyte, five and one quarter inch diameter fixed disk drive offered by Seagate Technology, Inc.; while "ST 412" identified the four disk, ten megabyte product from the same source. Those particular drives employed an open loop stepper motor head actuator, and consequently, the interface for each drive was selected to be similar to the interface for five and one quarter inch open loop stepper motor based floppy disk data storage devices, in terms of form factor, power supply requirements, and some of the control signals, such as read/write, stepping pulses for moving the transducer head to a concentric data track of interest, etc. Because of widespread, immediate acceptance, the ST 506 interface has become a de facto industry standard for small fixed rotating disk data storage devices.
One drawback of the ST 506 interface standard was its requirement that each unformatted concentric data track be capable of containing approximately 10,416 bytes of data transferrable at a 5 megabit data rate in modified frequency modulation (MFM) format without interruption. This arrangement permitted the external controller to format (divide) the track into data records having a wide variety of lengths and data storage capacities. Since the amount, data rate and recording method of the data were fixed, the only ways to increase the overall storage capability of the disk drive were to add disk surfaces, or to move the concentric data tracks closer and closer together. As the data tracks were moved closer together, thermal gradients, and tolerances imposed e.g. by the manufacturing process and materials used in construction rendered open loop stepper motor positioner systems less and less reliable. With the requirement for higher track densities on the disk came a need for more precise positional feedback information from the one or more disk surfaces.
The assignee of the present invention pioneered the concept of a low cost, higher capacity disk drive having a single data masked servo sector on a data surface of a disk which was hidden in a speed tolerance gap provided just before the user index marker was sent to the device controller over the interface. Coarse track boundary information was provided by a detent-providing stepping motor or by an optical position encoder linked to the actuator structure. This approach, discussed in the assignee's U.S. Pat. No. Re 32,075, issued on Jan. 28, 1986, works very well for increasing track densities in low cost fixed disk data storage devices while maitaining compatibility with the popular, open loop based disk drive interface standards, such as the ST 506 standard.
The only other practical approach for increasing aerial track density without restricting the format of the data to be stored in each track is to devote an entire disk surface to the servo control process. The additional or "dedicated" servo surface is typically prerecorded with concentric servo tracks which are followed by a servo head whose sole task is to provide track following control information during read and write operations of the disk file. A principal drawback of a dedicated servo surface is its higher cost: it not only requires that an extra data storage surface and head be provided exclusively for handling the servo information (high cost components in low cost devices), but also requires separate read channel electronics for the servo loop. Performance may be enhanced and costs still kept low if an optical position encoder or other detent providing mechanism is used in conjunction with a dedicated servo surface prerecorded with a non-phase-coherent servo pattern. One very satisfactory high capacity, low cost disk file which advantageously combines these features is described in the assignee's U.S. Pat. No. 4,516,177.
One other prior approach worth mentioning is that of an embedded servo sector disk file. That approach, followed for some time by integrated high capacity, high performance, high cost disk file subsystems, has been found to be useful in interspersing servo information with user data on a disk surface. One drawback of some implementations of this embedded servo sector prior approach has been, for example, the requirement that velocity of the head assembly be carefully controlled and limited to a maximum value during track seeking operations so that the head reading the servo information will be in position to pass over each servo sector during the seeking operation, thereby enabling the servo sector to be read and head position to be determined. This strict position control requirement prevented the servo from losing track of the instantaneous position of the head, but degraded servo performance by calling for velocity limits during seek.
Another drawback of the prior embedded sector servo approach was the requirement for very precise servo writing systems for writing the embedded servo data in a manner in which each magnetic change of phase or transition (bit) written as a servo datum for a data track was coherently related to all other servo data. This severe servo writing requirement, which involved precision to the level of ten or less nanoseconds, significantly added to the cost of manufacture and therefore the drive itself.
A further drawback of the prior embedded sector servo approach lay with the various layouts of servo information prerecorded onto each data storage surface. Those patterns did not always provide reliable absolute position information or track centering information, and rarely did such patterns provide quadrature information which is very helpful in order to determine the radial direction of head transducer movement.
One more drawback of the prior art related to the various ways in which defects in the storage media were taken into account in order to minimize the resultant degradation of capacity or performance of the data storage device.
As can be seen by this summary of prior developments, a hitherto unsolved need has arisen for a low cost, high performance and capacity rotating disk data storage device which overcomes the limitations and drawbacks of the prior art.